Prosecutors in the Los Angeles suburb responsible for a huge share of the nation’s wiretaps almost certainly violated federal law when they authorized widespread eavesdropping that police used to make more than 300 arrests and seize millions of dollars in cash and drugs throughout the USA.

The violations could undermine the legality of as many as 738 wiretaps approved in Riverside County, Calif., since the middle of 2013, an investigation by USA TODAY and The Desert Sun, based on interviews and court records, has found. Prosecutors reported that those taps, often conducted by federal drug investigators, intercepted phone calls and text messages by more than 52,000 people.